1. Field of the Invention
The present invention relates to a method for reducing the oxygen content of steel compacts while simultaneously controlling the carbon content thereof. More particularly, it relates to a process for controlling the oxygen and carbon levels in steel compacts by heating the same in a reducing atmosphere.
2. Description of the Prior Art
It is well known that steel powder compacts obtained by compacting powder-metallurgical steel normally contain oxygen at levels as high as 1,000 to 2,000 ppm. Therefore, it is essential that in order to obtain products of good quality, the oxygen content should be reduced to acceptable levels in the subsequent heating step, while the carbon levels are controlled in the steel compacts. A variety of processes including the RX, SRX, and ASRX processes are known in which steel compacts such as those shown in Table 1 are treated under such gaseous atmospheres prepared from paraffinic hydrocarbons as methane, propane, butane and the like. An AX gas process has also been proposed which employs an atmosphere of a mixture of hydrogen and nitrogen gas obtained by the decomposition of ammonia.
In these processes, reducing atmospheres such as hydrogen gas are used to deoxidize the steel powder compacts. During the deoxidation reaction the reducing constituents in the atmosphere employed tend to react with oxygen in the powder compacts, thereby causing the evolution of oxidative gases. The oxidative gases then react with carbon in the powder compacts, and decarburized layers are formed. As a result, it is difficult to control carbon concentration in the steel compacts, and undesirable decarburized layers are formed in the steel compacts which result in poor quality of the products.
In the processing of the steel compacts, various reactions between the reducing gases in the atmosphere and the steel powder compacts occur which are illustrated as follows: EQU CO+FeO=FE+CO.sub.2 (Deoxidation) (1) EQU CO.sub.2 +Fe(C)=Fe+2CO (Decarburization) (2) EQU H.sub.2 +FeO=Fe+H.sub.2 O (Deoxidation) (3) EQU H.sub.2 O+Fe(C)=Fe+CO+H.sub.2 (Decarburization) (4)
From the above reactions, it can be readily understood that decarburization reaction (4) is caused by the evolved water vapor. This reaction cannot be prevented even when an atmosphere of a low dew point (e.g., a dew point of -40.degree. C.) is used. To overcome this problem, it has been proposed to use RX gases as an atmosphere and to heat the steel powder compacts to a high temperature for long periods of time. In this technique, however, oxygen remains at high levels in the steel powder compacts, although the carbon content is maintained roughly at an expected value. In the situation where high temperatures are used to reduce the oxygen content, it is extremely difficult to obtain a proper carbon level with the atmosphere because of inherent design features of the generator. Thus, accurate carbon control in the steel products is almost impossible.
As is obvious from the above equations, the atmosphere over the steel compact should be made strongly reductive, and reactions (1) and (3) should be allowed to proceed smoothly, in order to lower the oxygen content of the steel powder compacts. In practice, however, the decarburization reactions shown by equations (2) and (4) proceed concurrently. As a result of this contradictory behavior, neither the formation of decarburized layers can be prevented, nor can satisfactory deoxidation be achieved.
Thus, in accordance with the prior art processes, it is quite difficult to achieve sufficient deoxidation and to prevent the formation of decarburized layers while maintaining the carbon content of the steel compacts at an optimum level. In fact, it has been impossible to produce powder metallurgical steel products of excellent quality from steel powder compacts containing oxygen in excess of 1,000 ppm.
A need, therefore, exists for a method by which steel compacts can be heat treated to deoxidize the same while minimizing the decarburization of the same.